桂西北天峨孤立碳酸盐岩台地晚古生代沉积特征与演化

Sedimentary Characteristics and Evolutionary Stages in Late Paleozoic of Tian'e Isolated Platform in Northwestern Guangxi

桂西北天峨碳酸盐岩台地形成于早泥盆世晚期,于早三叠世末最终消亡,长期被深水海槽盆地所围限。晚古生代其是右江盆地北缘一个长期持续发育的孤立碳酸盐岩台地,地层沉积特征主要受基底沉降、海平面波动和盆地内生碳酸盐沉积速率的控制,可划分为孤立台地和斜坡—盆地两大相区,共识别出局限台地相、半局限台地相、开阔台地相、台地边缘相、台地前缘斜坡相和盆地相6种沉积相类型。各时期岩相古地理之间具有明显的继承性和发展性,并建立相应的沉积地层序列及孤立台地沉积模式。天峨碳酸盐岩台地的发展演化可划分为孤立台地孕育阶段(D31—D2)、孤立台地成型阶段(D3)、孤立台地持续发展阶段(C1—P2)和孤立台地收缩淹没阶段(P3—T1)4大演化阶段,与右江裂谷盆地(D31—D3)—被动大陆边缘构造演化阶段(C1—T1)的进程相对应,反映了右江盆地晚古生代剧烈扩张、沉降,扬子台地边缘拉伸、破裂,微地块向盆地内部滑移的构造背景。

The Tian＇ e carbonate platform in Northwestern Guangxi, formed in the late period of Early Devonian and demised in the end of Early Triassic, was surrounded by deep sea trough during Late Paleozoic. It is a typical isolated carbonate platform in the northern part of the Youjiang Basin, and the stratigraphic and sedimentary characteristics of the Upper Paleozoic are mainly controlled by the base subsidence, sea-level fluctuation, as well as changes of the carbonate deposition rate in the basin. Two facies regions and six kinds of sedimentary facies can be identified, mainly comprising isolated platform facies region, slope-basin facies region, and restricted platform, semi-restricted platform, open platform, platform margin, front slope of carbonate platform and basin between platforms facies, respectively. Palaeogeography in different periods have obvious inheritance and development, so a related depositional model for the isolated platform can be established. The current study suggests that it experienced four evolutional stages, namely, embryonic stage （D3- D2 ） , forming stage （D3 ）, sustained development stage （ C1-P2 ） and atrophy-foundering stage （ P3-T1 ）, which correspond to the process of Youjiang Basin evolving from rifting basin （D1/3--D3 ） to passive continental margin （C1-T1） with intense expansion and subsidence, while the southern margin of the Yangtze Platform experienced extension and breaking splitting.